Wireless networks, such as networks operating under Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards, are spreading in their popularity and availability. With such popularity, however, comes the problem of communication channel management. Specifically, since many clients on the same wireless communication channel may share and compete for channel access, network performance typically depends on the number of clients that share the wireless communication channel within a given radio frequency neighborhood. Thus, it is advantageous to distribute clients to span across different wireless communication channels in a given radio frequency neighborhood.
Roaming generally refers to passing a client from one access point (AP) to another access point in a wireless local area network (WLAN) while the client remains connected to the WLAN. Note that, in some specific scenarios, a client may be handed over from one AP in one network to another AP in a different network of a different type.
In previous solutions to client roaming in WLAN, the client's roaming decision usually is based on the client itself. According to one conventional scheme, a client selects one of multiple APs in the network to associate with based on signal strengths (for example, as identified by Received Signal Strength Indication, RSSI) of packets received from the APs. Specifically, the higher a RSSI is for a packet received from an AP, the more likely it is for the client to select the AP to associate with. Hence, network devices such as access points do not have any control over the client's roaming decisions in this process.
Moreover, when a client moves from one AP to another AP, it is possible for the client to reach a location point where the signal strengths associated with the packets received by the two APs are equal. Such condition can cause the client to toggle between the two APs. For example, at time t1, the client may detect that the RSSI of a packet received from AP1 to be slightly higher than the RSSI of another packet received from AP2 in the same network. Thus, the client would associate with AP1, at time t1. Thereafter, at time t2, the client may detect that the RSSI of a packet received from AP2 to be slightly higher than the RSSI of another packet received from AP1. Thus, the client would associate with AP2 instead at time t2. Shortly after the client switch to associate with AP2, the client may soon detect that the RSSI of a packet received from AP1 to be higher than the RSSI of another packet received from AP2 again, and thus the client may associate back with AP1 at time t3.
According to an alternative implementation scheme, a client may have a predetermined high RSSI watermark and a predetermined low RSSI watermark. The client will decide to switch to associate with AP2 from AP1 if the RSSI of a packet received from AP2 is higher than the high RSSI watermark, and if the RSSI of a packet received from AP1 is lower than the low RSSI watermark. With the predetermined high and low watermarks, it is less likely for the toggling situation to rise. However, in certain scenarios, a toggling situation may still occur. For example, assuming that, at time t1, the client is at a location where the signals from AP2 are obstructed. Thus, the RSSI of a packet received from AP2 may be just slightly above the predetermined low RSSI watermark, but lower than the predetermined high RSSI watermark. Also, the RSSI of a packet received from AP1 is likely to be higher than the RSSI of the packet received from AP2. Thus, the client will remain associated with AP1 at the location at time t1. Further assuming that, at time t2, the client moves to a new location where signals from AP2 are not obstructed. The new location at time t2 is closer to AP2 and farther to AP1 than the original location at time t1. Thus, the RSSI of the packet from AP2 received at time t2 is likely to be above the predetermined high RSSI watermark; and, the RSSI of the packet from AP1 received at time t1 is likely to drop below the predetermined low RSSI watermark. Thus, the client will likely to associate with AP2. If the client subsequently moves between the old location and the new location, a toggling situation will result.
The above described scenarios are more likely to happen with high mobility devices, such as handheld devices, Voice-over-WLAN clients, etc. Usually, the high and low RSSI watermarks are predetermined. They are neither configurable on a per-device basis nor dynamically configurable. Moreover, the mixture of devices makes it difficult to configure the high and low RSSI watermarks for each device in order to improve operations in complex network deployments and to facilitate a client's roaming decisions.